1. Field of the Invention
The present invention relates to a process for producing a multilayer molded article. More particularly, the present invention relates to a process for producing a multilayer molded article comprising a thermoplastic resin and a skin material having good appearance, which is used as an automobile interior material and a part of an electric equipment.
2. Description of the Related Art
Hitherto, molded articles on which a skin material is laminated to improve their beauty, softness and the like so as to make their grade higher were widely used.
To produce such molded articles having the skin material, there is a process known which comprises molding a thermoplastic resin by one of the conventional molding methods such as injection molding and then molding the skin material onto the molded article by vacuum molding and the like or simultaneously molding the thermoplastic resin and the skin material wherein the skin material is adhered to the molded resin with an adhesive to laminate the skin material onto the molded resin.
Another known process comprises placing the skin material in a mold, closing the mold and then injecting the hot thermoplastic resin melt through a gate into the mold to laminate the resin onto the skin material, cooling the mold and removing the multilayer molded article is also known.
A further known process comprises placing the skin material between the upper and lower molds, supplying the thermoplastic resin melt while the molds are unclosed, closing the molds to integrate the thermoplastic resin and the skin layer together.
However, each of the conventional processes has its respective drawbacks.
Since the first process uses the adhesive to laminate the molded resin and the skin material, the steps become complicated and in turn a production cost of the molded article becomes high, and a solvent or a dispersant contained in the adhesive will pollute the environment.
In the second process, since the resin melt which is pressurized at a high temperature is injected into the mold, so that the resin melt contacts and presses the skin material strongly against the mold wall, the surface of the skin material is discolored, a pattern of the skin material is deformed or the appearance of a fabric with raising is deteriorated, due to flatting or flowing of the raising. Further, in some cases, the resin melt impregnates or penetrates into the skin material so that the commercial value of the molded article is impaired.
The drawbacks of the second process can be overcome by the third process to some extent so that, according to a type of skin material (a kind, quality, etc.), a good multilayer molded article can be obtained. However, the quality of the molded article is still unsatisfactory.
To overcome the above described drawbacks or to prevent deterioration of the softness of a skin material, it was proposed to interpose a plastic foam between the skin material and the thermoplastic resin or to use a packing material such as paper or a dense plastic foam as a heat insulator or a resin barrier (cf. Japanese Patent Kokai Publication Nos. 212343/1985 and 29436/1982). When the plastic foam alone is interposed between the skin material and the thermoplastic resin, the cells of the foam are broken by heat and pressure from the resin melt, whereby distinct uneveness is formed on a surface of the foam which contacts the skin material and the unevenness is transferred to the surface of the skin material so that not only the appearance, but also the feeling of the molded article are impaired. When the packing material which has stiffness and comparatively small elongation such as paper or dense plastic foam is interposed between the skin material and the thermoplastic resin, it may be partially broken or warp in the case of a molded article having a complicated shape, whereby the appearance of the article is impaired.
Further, it was proposed to use a skin material having a protective layer made of the same plastic as the thermoplastic resin of the article (cf. Japanese Patent Kokai Publication Nos. 71161/1979, 109759/1981 and 109760/1981). It was also proposed to use a skin material on a back surface of which a foam layer is adhered or welded and a packing material (cf. Japanese Patent Kokai Publication No. 212342/1985).
It is possible to maintain the appearance (or feeling) of the skin material to some extent by these methods, but further improvement is desired.